The present invention generally relates to thermionic energy conversion and, more particularly, to improved thermionic energy converters wherein an acceleration electrode and shaped surfaces of the emitter and collector produce a convergent/divergent electron flow, and the acceleration electrode doubles as a thermal radiation shield.
Thermionic energy converters are generally diode devices based on the Edison effect. A heated emitter emits electrons which are collected by a low-temperature collector. Any heat source may be used to heat the emitter, but solar furnaces show considerable promise. However, as a means of solar energy conversion, thermionic energy converters must compete with photovoltaic systems. While, in theory at least, thermionic energy converters would appear to be quite competitive with photovoltaic systems, especially in regard to high potential values of the power-to-mass ratio, in practice, considerable difficulties have been encountered in the development of thermionic energy converters.
A typical thermionic energy converter consists of two plane parallel electrodes separated by a high-vacuum or low-pressure gas. The high-temperature emitter has a high work function, and the low temperature collector has a lower work function. Electrons boil off the emitter, and some of them reach the collector to produce a net current flow through the device. A voltage difference equal to the difference in work functions of the emitter and collector is theoretically possible. When the space between the emitter and collector is a vacuum, a large portion of the electrons boiled off the emitter are prevented from reaching the collector by a negative space charge which builds up between the two electrodes. Very close spacing (.about.10 microns) of the electrodes is therefore essential to produce any appreciable current. The spacing and the space charge problems has led to the almost exclusive use of gas filling for most applications. A small amount of cesium in the diode envelope produces a vapor which alleviates the space charge problem somewhat, but this also has the property of altering the work functions of the emitter and collector, depending on the temperature and surface coverage.
Another disadvantage of the typical thermionic energy converter is thermal radiation from the hot emitter to the cooler collector. This radiation loss can be reduced by the use of less desirable combinations of emitter and collector work functions and temperature with considerable sacrifice in efficiency and/or power density.
The upper bound of efficiency for thermionic energy converters is dictated by the ideal Carnot cycle efficiency. Cesium-filled thermionic devices have been operated in the 20% efficiency range (up to about 35% of the Carnot cycle efficiency) for short periods, and in the 10-15% efficiency range with longer lifetimes. Thus, the promise of relatively high efficiency energy conversion in a practical device has not been realized.